In the modern society, rare earth elements are used in a wide variety of applications, for example, as rare earth magnets, phosphors, and electronic materials in nickel hydrogen batteries. With respect to the current supply of rare earth elements, a crisis of the rare earth resource is highlighted because the producers are limited, the price lacks stability, and the demand is expected to surpass the supply in the near future. For these reasons, many attempts are made to reduce the amount of rare earth element used and to develop a replacement. At the same time, it is desired to establish a recycle system for recovering rare earth elements as one valuable from in-process scraps produced during manufacture of products and municipal wastes like electric and electronic appliances collected from cities. Also there is an urgent need for the research and development of new rare earth mines.
Known methods for separating rare earth elements include column extraction (or solid-liquid extraction) using ion exchange resins, and solvent extraction (or liquid-liquid extraction) using metal extractants. Although the column extraction (or solid-liquid extraction) method is simple in apparatus and easy in operation as compared with the solvent extraction method, it is small in extraction capacity and discourages rapid treatment. The column extraction method is thus used in the removal of a metal when the concentration of a metal to be extracted in a solution is low, that is, when the metal to be extracted is present as an impurity, as well as in the waste water treatment. On the other hand, the solvent extraction (or liquid-liquid extraction) method needs a complex apparatus and cumbersome operation as compared with the column extraction method, but provides for a large extraction capacity and rapid treatment. Thus the solvent extraction method is often used in industrial separation and purification of metal elements. For the separation and purification of rare earth elements that requires efficient treatment of a large volume through continuous steps, the solvent extraction method capable of such efficient treatment is often used.
In the solvent extraction method, a water phase consisting of an aqueous solution containing metal elements to be separated is contacted with an organic phase consisting of an extractant for extracting a selected metal element and an organic solvent for diluting the extractant. Then the metal element is extracted with the extractant for separation.
Known metal extractants used in the art include tributyl phosphate (TBP), carboxylic acids (e.g., Versatic Acid 10), phosphoric acid esters, phosphonic acid compounds, and phosphinic acid compounds. A typical phosphoric acid ester is di-2-ethylhexylphosphoric acid (D2EHPA), a typical phosphonic acid compound is 2-ethylhexylphosphoric acid-mono-2-ethylhexyl ester (PC-88A by Daihachi Chemical Industry Co., Ltd.), and a typical phosphinic acid compound is bis(2,4,4-trimethylpentyl)phosphoric acid (Cyanex 272 by Cytec Industries). These extractants are commercially available and commonly used.
The separation efficiency of the solvent extraction method depends on a separation ability of the metal extractant, specifically a separation factor thereof. As the separation factor is higher, the separation efficiency of the solvent extraction method is higher, which enables simplification of separating steps and scale-down of the separation apparatus, making the process efficient and eventually leading to a cost reduction. A low separation factor, on the other hand, makes the separation process complex and poses a need for a large-scale separation apparatus.
Even PC-88A which is known to have a high separation factor for rare earth elements among the currently commercially available metal extractants has a low separation factor between elements of close atomic numbers, for example, a separation factor of less than 2, specifically about 1.4 between neodymium and praseodymium which are allegedly most difficult to separate among rare earth elements. The separation factor of this value is not sufficient for separation between neodymium and praseodymium. To separate them at an acceptable purity, a large-scale apparatus must be installed at the expense of cost. For more efficient separation of these elements, there is a desire for the development of a metal extractant having a higher separation factor than in the prior art and an extracting/separating method using the same.
Dialkyl diglycol amic acids are known from Patent Document 1: JP-A 2007-327085 as the metal extractant having a high separation factor with respect to rare earth elements, specifically light rare earth elements such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), and samarium (Sm). Using this extractant in solvent extraction, the extraction/separation step of rare earth elements, specifically light rare earth elements can be made more efficient. In fact, better results are obtained from the extraction/separation step of light rare earth elements using dialkyl diglycol amic acid on a laboratory scale.
When dialkyl diglycol amic acid was used as the metal extractant, satisfactory results were confirmed in a light rare earth element extraction/separation experiment which was conducted at a rare earth element concentration (CA: 0.01 mol/L≦CA≦0.7 mol/L) and a corresponding metal extractant concentration (CA: 0.1 mol/L≦C0≦1.5 mol/L) which were practical operating conditions of the rare earth element separating process and in a light rare earth element extraction/separation experiment using a countercurrent multi-stage mixer/settler of a practically operating apparatus.
The dialkyl diglycol amic acid exhibits a satisfactory separation factor in its performance as the metal extractant for separating light rare earth elements, as mentioned above, and its operating conditions have been surveyed. However, its synthesis has not been fully established.
The known method for synthesizing the dialkyl diglycol amic acid follows the reaction scheme below.
Herein R1 and R2 are each independently alkyl, and at least one is a straight or branched alkyl group of at least 6 carbon atoms.
First, diglycolic anhydride is suspended in dichloromethane. A secondary alkylamine in an amount slightly less than an equimolar amount to the diglycolic anhydride is dissolved in dichloromethane. The solution is mixed with the suspension at 0 to 30° C. As diglycolic anhydride reacts, the mixed solution becomes clear. The reaction is completed when the solution becomes clear. This is followed by removal of water-soluble impurities by washing with deionized water, removal of water with a dehydrating agent (e.g., sodium sulfate), filtration, and solvent removal. Recrystallization from hexane is repeated plural times for purification, yielding the desired product (Patent Document 1: JP-A 2007-327085).
This synthesis method uses as the reaction solvent dichloromethane which is one of the harmful substances listed in the Chemical Substance Examination Law, Labor Safety and Health Regulations, Air Pollution Control Act, Water Pollution Control Act, Pollutant Release and Transfer Register (PRTR) and the like in Japan. It is recommended to avoid the substance. In addition, since the solubility of the reactant, diglycolic anhydride is not so high, the synthesis reaction becomes a solid-liquid reaction and has a poor reactivity.
In fact, the above known synthesis method gives a yield of more than 90% because it is conducted only on a laboratory scale where the amount of synthesis is several grams. However, a prominent drop of yield occurs when the synthesis is enlarged to a scale of several kilograms or more. In fact, in a synthesis experiment conducted on a scale of several hundreds of grams, the yield decreases below 80%. Such a yield drop is unwanted.
Further, since diglycolic anhydride is a relatively expensive chemical, the price of dialkyl diglycol amic acid synthesized therefrom is at least 3 times the price of commercially available metal extractants. This method has a significant effect of enhancing process efficiency due to excellent separation capability, but does not lead to a cost reduction of the overall process because of the increased expense of metal extractant.